CO₂ condensation onto alkanes: Unconventional cases of heterogeneous nucleation

The classical picture invoked for heterogeneous nucleation is frequently that of a liquid condensing onto an immiscible solid particle. Here, we examine heterogeneous nucleation of CO₂ onto particles comprised of n-pentane or n-hexane under conditions where CO₂ should be a solid and the seed particles may be liquid or solid. Although CO₂ condensed under all but one of the six conditions investigated, these experiments do not easily fit into the framework of standard heterogeneous nucleation experiments. Rather they explore unconventional regimes of heterogeneous nucleation in which the state of the seed particle may both affect whether deposition can proceed, and, in turn, be influenced by the presence of the condensing species. The work complements the earlier work of Tanimura et al. [RSC Adv., 2015, 5, 105537-105550] that investigated CO₂ condensation onto ice nanoparticles, by using seed particles comprised of non-polar compounds that form and freeze under conditions where CO₂ is already supersaturated with respect to the solid ice. In some cases, the conditions for seed formation approach the limit of homogeneous CO₂ nucleation. Vibrational spectroscopy measurements help pinpoint where CO₂ starts to condense. Furthermore, these IR measurements suggest that the n-alkanes never freeze in the presence of CO₂, even if the temperatures are well below those required for them to freeze when CO₂ is absent. Over the temperature range 65 < T/K < 140, the conditions corresponding to the onset of CO₂ heterogeneous nucleation on pre-existing seed particle almost all lie very close to the extrapolated vapor-liquid equilibrium line of CO₂ for a broad range of seed materials.